Quadrupole mass analyzers have conventionally been used as flow-through devices, i.e., a continuous stream of ions enter and then exit the quadrupoles. More recently, however, the same quadrupole mass analyzer has been used as a combined linear ion trap and mass analyzer. That is, the linear ion trap accumulates and constrains ions within the quadrupole volume. The linear ion trap is characterized by an elongate multi-pole rod set in which a two dimensional RF field is used to constrain ions radially and DC barrier or trapping fields are used to constrain the ions axially. After a suitable fill time, the trapped ions are then scanned out mass dependently, for example, using a radial or axial ejection technique. Examples of quadrupole mass analyzers which combine ion trapping and mass analysis functions are described, inter alia, in U.S. Pat. No. 5,420,425 to Bier at al.; U.S. Pat. No. 6,177,668 to Hager; or in co-pending U.S. patent application Ser. No. 10/310,000, filed Dec. 4, 2002 and assigned to the assignee of the instant application. Each of these documents is incorporated herein by reference.
In such quadrupole mass analyzers, the mass scan sometimes reveals ghost peaks, i.e., satellite peaks that appear adjacent to the main peak, making the mass scan questionable. An example of this is shown in FIG. 1A, where a mass scan 78 features a main mass peak 82. The satellite peak 80, on the low side of the main peak 82, is a ghost peak or artifact. The small peak 84, on the high side of mass peak 82, is a legitimate isotope peak. These spectrograms were taken using a commercially available standard solution manufactured by Agilent™, product no. ES Mix G2421A, diluted in acetonitrile and water. Artifacts of these types have been observed on a number of mass spectrometers when a quadrupole rod set has been operated as a combined ion trap and mass analyzer. As mass increased, the severity of the artifact peaks increased in that the mass separation increased with mass, i.e., the problem was worst at high mass. The problem was also much more evident at slow scan speeds (e.g., 250 Da/s) when the resolution is the best. The age of the equipment and the length of the rods was also a factor. Depending on the parametric conditions, primarily the barrier potential on an end section member such as an exit lens used to trap ions axially, the artifact peaks could be minimized but at the expense of the main peak intensities. Again depending on the instrument and how it is set up the artifact peak can be either on the high or low mass side of the main peak.